Square battery container, method of manufacturing the container, and square battery using the container

ABSTRACT

A prismatic battery case with high dimensional precision is manufactured, while pursuing an improvement in productivity, with a manufacturing method of a prismatic battery case that includes a first step for molding an intermediate cup body ( 8 ) by impact molding a pellet ( 7 ) of prescribed shape, and a second step for molding a prismatic battery case ( 9 ) with a cross section of substantially rectangular shape by DI processing the intermediate cup body ( 8 ). The DI processing conducts drawing and ironing continuously, in one action.

TECHNICAL FIELD

The present invention relates to a prismatic battery case that is usedas an outer case for various prismatic batteries, a manufacturing methodfor manufacturing this prismatic battery case by the DI (Drawing andIroning) method, and a prismatic battery that is constituted by usingthe prismatic battery case made by this manufacturing method.

BACKGROUND ART

A lithium-ion rechargeable battery has advantages in that it is not onlyoutstanding in energy density per unit volume, but also in energydensity per unit weight, among all the battery systems in practical useat the present time. The energy density per unit volume is used as anindex of reduction in size of a battery, and the energy density per unitweight is used as an index of reduction in weight of a battery. Theenergy density of a battery is determined mainly by the battery activematerial of the positive and negative electrodes, which constitute thepower generation elements. However, reduction in size and weight of thebattery case, which accommodates the power generation elements, alsobecomes important factors. In other words, when a battery case is madethin, it is possible to accommodate more battery active materials into abattery case with the same outside shape, and improve energy density pervolume of the battery as a whole. It is also possible to reduce theweight of the battery as a whole, and improve energy density per weight,when the battery case is formed by a light material.

In the trend of the batteries mentioned above, a prismatic battery usinga thin prismatic battery case as its outer case is regarded especiallyimportant, because it is suited in making an equipment thinner, and alsobecause its efficiency in utilizing space is higher in comparison tocylindrical batteries. For conventional manufacturing methods of theprismatic battery case, there have been adopted the so-called transferdrawing method, which manufactures battery cases with substantiallyrectangular cross sectional shapes, by repeating deep drawing ten totwenty times with the transfer press machine. There have also beenadopted a method that uses impact molding, using aluminum as itsmaterial.

However, in the manufacturing method of the prismatic battery case thatuses the transfer drawing method, productivity is very low with onlyabout twenty pieces per minute, for example, because the deep drawinghas to be repeated ten to twenty times. Furthermore, it has a drawbackin that the cost rises, because the die for multistage drawing becomescomplicated, in addition to there being many steps. In the transferdrawing method, the thickness of the battery case material is madethinner by repeating the deep drawing, when making it thinner withobject of making the capacity larger by raising the energy density pervolume. Hence, a drawing-out and pushing-in of the punch is needed thesame number of times as the number of drawings, and the diameter of thepunch has to be made smaller each time, and the clearance between thepunch and the die also has to be made smaller each time. Thus, it needsto be made the same thickness as the side face, from the thick portionat the circumference of the bottom part, and the deep drawing to fulfillthis is very difficult. What is worse, the prismatic battery cases madefrom these processes lack strength at the circumferences of the bottompart, and there is a problem in that it is not possible to maintain aprescribed pressure resisting strength when functioning as a battery.

Japanese Patent Laid-Open Publication No. 2000-182573 discloses aprismatic battery case with its thickness of the plate portions on thelonger sides configured to be thicker than the thickness of the cornerportions. Japanese Patent Laid-Open Publication No. Hei. 6-52842discloses a prismatic battery case with its thickness of the plateportions on the longer sides configured to be thicker than the thicknessof the plate portions on the shorter sides. These prismatic batterycases are able to prevent expansion and deformation of the plateportions on the longer sides, when the pressure inside the batteryrises. However, the capacity to accommodate the power generationelements becomes smaller, because the thickness of the plate portions onthe longer sides are made to be thick. Since these portions have thelargest surface area, it is not possible to pursue an improvement inenergy density per volume or energy density per weight.

Japanese Patent Publication No. Hei. 7-99686 and Japanese PatentLaid-Open Publication No. Hei. 9-219180 disclose a battery case thatpursues a reduction in internal resistance when functioning as abattery, by forming fine reinforcements vertical to the bottom face,inside the case, to pursue an increase in contacting area with the powergeneration elements. However, it is almost impossible to achieve afunction that prevents expansion and deformation when the inner pressureof the battery rises, by using these vertical reinforcements of thebattery case. Japanese Patent Laid-Open Publication No. Hei. 7-326331discloses a prismatic battery case that has the thickness of the cornerportion configured to be thicker than the thicknesses of the plateportion on the longer side and the plate portion on the shorter side,which are straight-line portions. This prismatic battery case canimprove the efficiency to accommodate the power generation elements.However, it cannot prevent the expansion and deformation of the thinplate portions on the longer sides, being reinforced only by thethickened corner portion.

On the other hand, when manufacturing a prismatic battery case usingimpact molding, the prismatic battery case is formed by crushing apellet, which serves as a battery case material, with a punch, andpressing and spreading the material out through the gap between thepunch and the die, so that it spreads out along the outer peripheralsurface of the punch. Productivity is improved in comparison to thetransfer drawing method, but the precision of the dimensions is verybad, and the strength at its side portions will be insufficient when itis made thin. When a prismatic battery case is particularly functioningas a battery, and the pressure inside the battery rises, the deformationis bigger than that in a cylindrical battery case, which has a stableshape, and the plate portions on the longer sides with broad areasdeform so as to expand into a cylindrical shape, which is a more stableshape. Consequently, there is a fear of the equipment being damaged by aleak of electrolyte solution, or by a short circuit of the powergeneration elements. For this reason, it is necessary to have a shapethat reluctantly sacrifices reduction in thickness and weight, in orderto maintain enough strength to positively prevent a deformation when thepressure inside the battery rises, in manufacturing a prismatic batterycase by impact molding. Thus, it is not possible to pursue animprovement in energy density per volume or energy density per weight,in this case.

For other manufacturing methods of the prismatic battery case, JapanesePatent Laid-Open Publication No. Hei. 6-333541 discloses a method, inwhich a prismatic tube and a bottom plate are molded separately, and thebottom plate is joined hermetically to the bottom part of the prismatictube by laser beam welding. However, in this manufacturing method, thenumber of steps are not reduced so much in comparison to the transferdrawing method, and troublesome operations intervene, such as the stepthat accurately positions the prismatic tube and the bottom plate, orthe step that conducts the laser beam welding. Accordingly, it is notpossible to pursue an improvement in productivity. Further, it is notpossible to obtain a prismatic battery case that satisfies both of thecontradicting requirements, high energy density by reduced thickness andweight, and pressure resisting strength that prevents deformation duringa rise in inner pressure within the battery, in this manufacturingmethod.

A DI method is used for the manufacturing method of a battery case of acylindrical battery. This DI method makes it possible to manufacture abattery case that maintains a prescribed pressure resisting strength,while making it possible to pursue an improvement in energy density pervolume by reducing thickness. The DI method also makes it possible tomanufacture with high productivity. This DI method is a method thatconducts drawing and ironing on an intermediate cup body continuously,and in one action, and a battery case with a prescribed cylinder shapeis manufactured using this method. This intermediate cup body ismanufactured by deep drawing of a pressing machine. There are followingadvantages in this method compared to the transfer drawing method, andthe extent of its use is enlarging. The advantages include animprovement in productivity with the reduction in the number of steps,an improvement in dimensional precision such as thickness, animprovement in energy density corresponding to the reduction in weightand the increase in capacity, which are both due to a reduction inthickness of the peripheral side walls of the case, and a reduction instress corrosion.

Therefore, it is conceivable to manufacture a prismatic battery caseusing the DI method mentioned above. When manufacturing a cylindricalbattery case using the DI method, it is processed between twogeometrically similar shapes from an intermediate cup body having across section of circular shape to a battery case having a cross sectionof circular shape. The material flows uniformly during the processing,and deformation is done smoothly, because the thickness of the wholeperipheral wall is reduced uniformly, in the ironing step of the DIprocessing. In contrast, when manufacturing a prismatic battery caseusing the DI processing, it is processed between two geometricallynon-similar shapes from an intermediate cup body having a cross sectionof circular shape to a battery case having a cross section ofsubstantially rectangular shape. This makes the flow of materialnon-uniform, and thickness deviation, shearing, and cracks all caused byeccentricity are easily induced. This also makes processing difficult incorrespondence to the concentration of stress, because the processingstress acting during the molding is not uniform, and molding with highprecision also becomes difficult, because it is not possible to conducta stable processing. In particular, cracks and ruptures are liable to beinduced at the plate portions on the shorter sides, which have smallsurface areas, and there arises a problem that portions with distortedshapes are generated.

Japanese Patent Laid-Open Publication No. Hei. 10-5906 also discloses amanufacturing method of a prismatic battery case. In this method, afirst intermediate cup body is molded by drawing, and then a secondintermediate cup body is molded by repeating the drawing on theperipheral side wall portion of the first intermediate cup body, pluraltimes. Lastly, the second intermediate cup body is processed by impactextrusion (impact molding) to adjust the thickness to be a prescribedvalue at the bottom plate portion and the corner portion. However, thenumber of steps increases in this manufacturing method, with drawing,multi-stages of DI processing, and impact molding being necessary.Adjustment of the thickness at the bottom plate portion and theperipheral side wall portion also becomes difficult, because thethickness at the bottom plate portion is adjusted to be a prescribedvalue by impact molding in the last step. It is not possible to obtain aprismatic battery case having a shape with each of the portionsconforming to prescribed thicknesses, with high precision. As opposed tothis method, the present applicant has already proposed a manufacturingmethod that can manufacture a prismatic battery case that has highenergy density, and a prescribed pressure resisting strength using theDI method. In the first step of this manufacturing method, a hoopmaterial is punched and a battery case material 1 of oval shape isformed, as shown in FIG. 5A. Then, this battery case material 1 isprocessed by deep drawing to mold a first intermediate cup body 2 with across section of substantially elliptical shape similar to the circularshape, as shown in FIG. 5B. After that, the first intermediate cup body2 is subjected to multi-stages of continuous redrawing of the secondstep, which uses drawing press machines. After being subjected to thesecond step, this first intermediate cup body 2 is molded into a secondintermediate cup body 3, which has a cross section of substantiallyelliptical shape, and whose ratio of the minor-axis/major-axis issmaller than that of the cross sectional shape of the first intermediatecup body 2, as shown in FIG. 5C. Lastly, the second intermediate cupbody 3 is subjected to the DI processing, which conducts drawing andironing in a continued state, in the third step. This secondintermediate cup body 3 is molded into a prismatic battery case 4, whichhas a shape with a cross section of substantially rectangular shape, anda shape that has the thickness at the plate portion on the shorter side4 a thicker than the thickness at the plate portion on the longer side 4b, as shown in FIG. 5D.

With this manufacturing method, it is possible to manufacture aprismatic battery case 4 of desired shape in three steps. Productivityis improved remarkably in comparison to the conventional transferdrawing method, and it is also possible to obtain a prismatic batterycase 4 with high dimensional precision in thickness or the like by usingthe DI method. However, there are problems to be solved in thismanufacturing method. That is, ruptures and cracks are induced, whenattempting to manufacture a prismatic battery case by directly DIprocessing the first intermediate cup body 2. This is because an attemptis made to DI process the cup body 2 with a cross section ofsubstantially elliptical shape similar to the circular shape into a casewith a cross section of substantially rectangular shape. For thisreason, an intervention of the second step is needed. In this secondstep, the dimension in the minor axis direction is gradually shortenedby drawing, and the material of the deformation corresponding to thisdrawing is made to flow so that it escapes in the major axis direction.Moreover, the major axis side is modified by being shortened to aprescribed dimension. Consequently, the number of steps increases,because multi-stages of redrawing are to be conducted in the secondstep.

An object of the present invention is to provide a prismatic batterycase having high energy density and a prescribed pressure resistingstrength, and a prismatic battery using this prismatic battery case. Itis also an object of the invention to provide a manufacturing method ofthe prismatic battery case, which makes it possible to obtain thisprismatic battery case with high dimensional precision, and with ease,while reducing the number of steps by using the DI method and pursuingan improvement in productivity at the same time.

DISCLOSURE OF THE INVENTION

The present invention made to achieve the above-mentioned objects is amanufacturing method of a prismatic battery case, characterized byhaving a first step for molding an intermediate cup body by impactmolding a pellet of a prescribed shape, and a second step for molding aprismatic battery case with a cross section of substantially rectangularshape by DI processing the intermediate cup body, the DI processingconducting drawing and ironing continuously, and in one action.

With the manufacturing method described above, an intermediate cup bodywith a cross section of substantially rectangular shape is manufacturedin one action by impact molding that is able to manufacture a randomshape in one action. This intermediate cup body is then processed into aprismatic battery case by the DI method. Productivity is improvedremarkably with the number of steps reduced significantly, because thereis only the first step and the second step, both of which are able tomold with one stroke of movement of the punch. It is also possible tomanufacture a prismatic battery case of desired shape easily and withcertainty, and the precision of dimensions such as thickness isimproved, because the intermediate cup body is DI processed.Consequently, it is possible to manufacture a prismatic battery casewith sufficient pressure resisting strength, while making the thicknessas thin as possible.

Another aspect of the present invention is a prismatic battery case,characterized in that the case is formed by impact molding a pellet of aprescribed shape to make an intermediate cup body to be a prismaticshape with a bottom, and having a cross section of rectangular shapethat has thicknesses of a plate portion on the longer side, a plateportion on the shorter side, and a corner portion to have sizesascending in that order, and subjecting the intermediate cup body to DIprocessing which includes drawing and ironing continuously in one actionto have a shape with a cross section of rectangular shape, and thethicknesses of the plate portion on the longer side, the plate portionon the shorter side, and the corner portion to have sizes ascending inthat order.

When the pressure inside the battery rises, the force that tries toexpand and deform the plate portion on the longer side in the outwarddirection, and the force that tries to sink the plate portion on theshorter side in the inward direction are both acted upon the casesimultaneously. However, with this constitution, the plate portion onthe shorter side, which is thicker than the plate portion on the longerside, effectively prevents the plate portion on the longer side fromexpanding and deforming in the outward direction. The plate portion onthe longer side tries to expand and deform in the outward directionusing the corner portion just like a fulcrum. Hence, the corner portion,which is made even thicker than the plate portion on the shorter side,effectively prevents the plate portion on the longer side from expandingand deforming in the outward direction. Consequently, although thethickness of the plate portion on the longer side is configured to bethe thinnest, expansion and deformation of the plate portion on thelonger side in the outward direction with the rise in pressure withinthe battery is prevented and sufficient pressure resisting strength ismaintained. The capacity to accommodate the power generation elements ismade bigger by having the thickness of the plate portions on the longersides, which have the largest surface area among the peripheral wallportions, made the thinnest. In particular, when forming convexprotruding portions of lattice form on the plate portion on the longerside, the thickness at the plate portion on the longer side can be madeas thin as possible, because expansion and deformation of the case iseffectively restrained by the convex protruding portions. Even when thethickness of the corner portion is made thicker by being made a shapeprotruding in expansion in the inward direction for a distance that isthe same as the gap generated between itself and an electrode group tobe accommodated in the battery case, there is no reduction in thecapacity to accommodate the electrode group.

Another aspect of the invention is a manufacturing method of a prismaticbattery case, characterized by having a first step for molding anintermediate cup body by impact molding a pellet of a prescribed shape,and a second step for molding a prismatic battery case with a pluralityof convex protruding portions formed in lattice form arrangement. Theseconvex protruding portions protrudes in expansion and extends in linearform on the inner surface of the case and on at least the plate portionson the longer sides, so as to be thick in a thickness direction of thecase. This is done by subjecting the intermediate cup body to the DIprocessing using a DI punch, which conducts drawing and ironingcontinuously, in one action. The DI punch has processing grooves formedin lattice form, on at least surfaces on the longer sides of a prismaticplate with a cross section of rectangular shape.

With this manufacturing method, productivity is improved remarkably byhaving the number of steps reduced significantly. It is also possible tomanufacture a prismatic battery case of desired shape easily and withcertainty, and the precision of dimensions such as thickness isimproved, because the intermediate cup body is DI processed. It ispossible to manufacture a prismatic battery case with sufficientpressure resisting strength, while making the thickness as thin aspossible. In the DI processing of the second step, a portion of thematerial on the inner surface side of the intermediate cup body getsinto the processing grooves of the DI punch, while being subjected toplastic deformation. This makes the material on the inner surface sideof the intermediate cup body move in unity with the DI punch, because itis put in a state with resistance added between itself and the DI punch.This also makes the material on the outer surface side of theintermediate cup body to be ironed mainly by the die. Consequently, itis possible to make the flow of material smoother, by restraining thematerial surplus phenomenon between the die and the DI punch fromhappening. It is also possible to restrain the processing speed at theplate portion on the longer side, and make the processing speed constantas a whole, when the convex protruding portions are formed on only theplate portion on the longer side of the prismatic battery case. As aresult, it is possible to manufacture a prismatic battery case having auniform thickness, without any corrugated shape on either the innersurface or the outer surface of the case. The manufactured prismaticbattery case is made to be as thin as possible, but it has very strongpressure resisting strength, because it has strength to effectivelyrestrain an expanding deformation, with the lattice formed convexprotruding portions functioning just like reinforcement cross-pieces.

Another aspect of the invention is a prismatic battery, characterized inthat a power generation element composed of an electrode group andelectrolyte solution is accommodated inside a prismatic battery casemanufactured by the above-mentioned manufacturing method, and an openingof this case is liquid sealed with a sealing plate.

With this constitution, the prismatic battery case is manufactured in asmall number of steps, and productivity is improved accordingly. Theprismatic battery case also has sufficient pressure resisting strength,while pursuing an improvement in energy density per volume, by makingthe thickness of the prismatic battery case, which is formed with highprecision, to be as thin as possible.

With this constitution, the plate portion on the longer side, which isliable to expand and deform when the pressure within the battery rises,is restrained from expanding and deforming by the convex protrudingportions that function just like reinforcement crosspieces. Thus, itbecomes possible to form the thickness to be as thin as possible, to beless than 0.25 mm for example, and the capacity to accommodate the powergeneration elements becomes larger, and it is possible to pursue highenergy density.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A through FIG. 1C are schematic longitudinal sectional viewsshowing a first step in a manufacturing method of a prismatic batterycase according to a first embodiment of the present invention, insequential order;

FIG. 2 is a schematic longitudinal sectional view of a second step inthe same embodiment;

FIG. 3A and FIG. 3C are perspective views of pellets in the embodiment,FIG. 3B and FIG. 3D are perspective views of intermediate cup bodies,FIG. 3E is a perspective view of a prismatic battery case, and FIG. 3Fis a longitudinal sectional view of another prismatic battery case;

FIG. 4 is a longitudinal sectional view showing a prismatic batteryconstituted by using the prismatic battery case of the embodiment;

FIG. 5A is a plan view of a battery case material according to amanufacturing method of a conventional prismatic battery case, FIG. 5Bis a perspective view of a first intermediate cup body, FIG. 5C is aperspective view of a second intermediate cup body, and FIG. 5D is apartially cutaway perspective view of the prismatic battery case;

FIG. 6 is a schematic cross sectional view of a second step of amanufacturing method of a prismatic battery case according to a secondembodiment of the present invention;

FIG. 7A is a perspective view showing a DI punch used in the second stepof the embodiment, and FIG. 7B is an enlarged view of the VIIB portionin FIG. 7A;

FIG. 8 is a sectional view of a portion showing the case manufacturingprocess in the second step of the embodiment;

FIG. 9A is a perspective view showing the longitudinal sectional shapeof a prismatic battery case of the embodiment,

FIG. 9B is a perspective view showing a portion of the inner surface ofthis battery case in enlarged form, and

FIG. 9C is an enlarged sectional view of a portion of this battery case;and

FIG. 10A is a view of an intermediate cup body manufactured by the firststep the embodiment, seen from the opening thereof, and

FIG. 10B is a plan view of the prismatic battery case manufactured bybeing subjected to the second step, seen from the opening thereof.

BEST MODE FOR CARRYING OUT THE INVENTION

Preferred embodiments of the present invention will be described indetail hereinafter, with references to the drawings. First,manufacturing steps in a manufacturing method of a prismatic batterycase of a first embodiment will be explained briefly. In thismanufacturing method of the prismatic battery case, a pellet 7, which isthe battery case material, is formed to be an oval shape as shown inFIG. 3A, or substantially a rectangular shape as shown in FIG. 3C, inthe first step shown in the schematic longitudinal sectional views FIG.1A through FIG. 1C. Both of these shapes are plan view shapescorresponding to the outline of the cross sectional shape of theprismatic battery case to be manufactured. An intermediate cup body 8with a cross section of substantially elliptical shape with a smallminor-axis/major-axis ratio, as shown in FIG. 3B, or an intermediate cupbody 8 with a cross section of substantially rectangular shape, as shownin FIG. 3D, is formed by impact molding this pellet 7. A prismaticbattery case 9 of desired shape is manufactured by DI processing thisintermediate cup body 8 in the second step. The prismatic battery case 9is shown in a partially cutaway perspective view in FIG. 3E, and thesecond step is shown in a schematic longitudinal sectional view in FIG.2. The first and second steps will be described in detail and insequence, hereinafter.

When the pellet 7 is made an oval shape, it is possible to prevent acrushing of shape caused by the stress induced during impact molding,and to mold the intermediate cup body 8 into a desired shape, withcertainty. On the other hand, when the pellet 7 is made substantially arectangular shape, it is possible to form a intermediate cup body 8 withan outer shape close to the prismatic battery case 9 to be formed. Thus,the burden of processing is reduced, when DI processing thisintermediate cup body 8 to manufacture a prescribed prismatic batterycase 9.

FIG. 1A through FIG. 1C shows a pressing machine that conducts impactmolding in the first step, and a die 11 is fixed to a die holder 10 inthis machine. A pellet 7, which is the battery case material shown inFIG. 3A or 3C, is supplied to a processing hole 11 a on this die 11.Aluminum or aluminum alloy is used as the material for the pellet 7,because weight reduction of the prismatic battery case 9 to bemanufactured is pursued, and because it has malleability that isrequired by the impact molding in this first step. It is especiallypreferred that the aluminum alloy includes manganese. To be specific, analuminum alloy with a chemical composition of A1000 through A5000 inH4000 of the JIS standard is used. This type of aluminum alloy hasmalleability very much suited for impact molding, and it is alsopossible to achieve a preferable work hardening after the molding. It ismore preferable to use an aluminum alloy with a chemical composition No.A3000, and a battery case with excellent characteristics is achieved bythis alloy.

The pellet 7 is formed into an oval shape with its plan view shape madeto be an ellipse close to the cross sectional shape of the prismaticbattery case 9 to be manufactured, as shown specifically in FIG. 3A, orinto substantially a rectangular shape, as shown in FIG. 3C, by punchinga battery case material composed of aluminum or aluminum alloy describedabove. Annealing treatment is given to the pellet 7 with such a shapefor half an hour to three hours (preferably around one hour) attemperatures of 250 to 400 degrees centigrade. This annealing treatmentcan be given to the battery case material before punching the pellet 7,but it is preferred that it be given to the pellet 7 obtained by thepunching.

When the pellet 7 is inserted into the processing hole 11 a of the die11, a punch 13 held by a punch holder 12 is moved near to the die 11side, and then driven into the processing hole 11 a of the die 11. Thismakes the pellet 7 crushed by the punch 13, and it is cast so as tostretch up along the outer peripheral surface of the punch 13, whilebeing spread out so as to be pushed into the gap between the punch 13and the wall of the processing hole 11 a.

In this impact molding, a jagged surface 13 a formed on the front-endface of the punch 13 bites into the pellet 7, when it comes into contactwith the pellet 7. This holds the punch 13 at a prescribed relativelocation with regard to the pellet 7 without any deviation in location,in the subsequent impact molding processes. As a result, the material ofthe pellet 7, which deforms in correspondence to the progress of theprocessing, flows uniformly and smoothly to the circumference of thepunch 13. This makes it possible to mold a high precision intermediatecup body 8 without any thickness deviation.

It is preferred that the jagged surface 13 a be formed as a mesh formlike a knurling. The jagged surface 13 a is not always necessary, and itis possible to obtain an intermediate cup body of substantially thedesired shape by impact molding with a punch of flat front-end face.When the punch 13 has finished moving the prescribed stroke, anintermediate cup body 8 is molded, having the shape shown in FIG. 3B orFIG. 3D. There is a jagged surface 8d formed on the bottom face of thisintermediate cup body 8, with the jagged surface 13 a of the punch 13transcribed onto it. This intermediate cup body 8 is obtained by impactmolding the pellet 7 which has been given the annealing treatment tohave good extensibility. Thereby, the intermediate cup body 8 has evenless dispersion in the thickness of its side face portions.

The intermediate cup body 8 is substantially the same shape as thesecond intermediate cup body 3 shown in FIG. 5C, and has a shape with across section of substantially a desired ellipse or rectangular shape.That is, in this manufacturing method, an intermediate cup body 8 equalto the second intermediate cup body 3 is molded in one action byconducting the impact molding of the first step. This secondintermediate cup body 3 was manufactured by being subjected to the firststep that conducts the deep drawing, and the second step that conductsthe redrawing in the manufacturing method of the prior applicationmentioned before. However, this intermediate cup body 8 is formed inonly one step, which is the impact molding. Thus, there are somedistorted deformations, but these can be sufficiently modified by the DImethod in the second step, which will be mentioned hereinafter, andthere is no problem whatsoever.

Then, the punch 13 that has finished moving the prescribed stroke partsfrom the die 11, and moves towards its original position as shown inFIG. 1C. At this moment, the molded intermediate cup body 8 is pulledout of the processing hole 11 a by the punch 13 in a state adhering tothe punch 13, and then detached from the punch 13 by a stripper 14.

When conducting DI processing in the second step, which will bementioned hereinafter, a stress that makes the material try to gather inthe center direction is acted on, and the obtained battery case tends tobecome a crushed shape. In this embodiment, in contrast to thisphenomenon, an intermediate cup body 8 with a cross section ofsubstantially an ellipse or rectangular shape is formed by impactmolding a pellet 7 that is formed to be an oval or rectangular shapebeforehand, and then this intermediate cup body 8 is DI processed.Thereby, a battery case of desired shape is molded with the crushing dueto stress being prevented, or the burden of processing reduced.

The thickness of the intermediate cup body 8 can be configured withoutany limitation by configuring a gap between the punch 13 and the wall ofthe processing hole 11 a of the die 11. In an intermediate cup body 8having a thin side face portion, the burden in processing during the DIprocessing in later steps is small, but the controlling to obtain abattery case of a prescribed thickness becomes difficult. Conversely, inan intermediate cup body 8 having a thick side face portion, thecontrolling to obtain a battery case 9 of a prescribed thickness iseasy, but the burden in processing during the DI processing becomesbigger. In this situation, when the intermediate cup body 8 is formed tohave a shape with the following ratios, the burden in processing duringthe DI processing of this intermediate cup body 8 is reduced and thecontrolling to obtain a battery case 9 of a prescribed thickness becomeseasy. The ratio of the thickness of the plate portion on the longer sideto the thickness of the plate portion on the bottom is made to be 0.6 to1.3, and the ratio of the thickness of the plate portion on the shorterside to the thickness of the plate portion on the bottom is made to be1.0 to 1.8. In particular, it is possible to prevent an occurrence ofproblems such as pulling apart of the plate portion on the longer side.

Even when the intermediate cup body 8 is obtained by impact molding abattery case material or a pellet 7 that has been given the annealingtreatment mentioned above, it is preferred that the intermediate cupbody 8 be given annealing treatment again before DI processing. Thisannealing treatment is performed for about half an hour to three hours(preferably around one hour), at temperatures of 250 to 400 degreescentigrade. With this treatment, the intermediate cup body 8 has thework hardening induced during the impact molding relieved, and has theextensibility of the material made better, and it is made into a statesuited for the DI processing in the next step.

This intermediate cup body 8 becomes a prismatic battery case 9 ofdesired shape by being subjected to the DI processing in the secondstep, shown in FIG. 2. This DI processing conducts one stage of drawingand three stages of ironing all in one action, using a drawing andironing machine. This drawing and ironing machine is constituted bybeing provided with an intermediate product transfer unit 17, a diemechanism 18, a stripper 19, and the like. There is a drawing die 18A, afirst ironing die 18B, a second ironing die 18C, and a third ironing die18D arranged and installed on the die mechanism 18. These dies 18Athrough 18D are arranged in series, so that they are concentric with thecentral axis of the DI punch 20.

The intermediate product transfer unit 17 transfers intermediate cupbodies 8 to the molding location, in succession. The intermediate cupbody 8 transferred and positioned to the molding location is drawn bythe drawing die 18A, so as to have its shape corresponding to the outershape of the DI punch 20. This is done by the pushing movement of the DIpunch 20 driven by a flywheel (not shown). The cup body that hasfinished passing through this drawing die 18A is deformed to have thedimensions in the major axis direction and the minor axis direction tobe a little smaller than the intermediate cup body 8, and also deformedto have a long body portion. It is also molded to have its cross sectionsubstantially an elliptical shape, close to the substantiallyrectangular shape of the cross section of the desired prismatic batterycase 9, but there is no change in its thickness.

Then, the cup body that has finished passing through the drawing die 18Ais subjected to the first stage of ironing by the first ironing die 18B,with the progress of the pushing movement of the DI punch 20. This makesthe peripheral side portion flatten out and makes its thickness thinner,and also increases the hardness by work hardening. The cup body that hasfinished passing through the first ironing die 18B is successivelysubjected to the second and third stages of ironing conducted by thesecond ironing die 18C and the third ironing die 18D, with the furtherprogress of the pushing movement of the DI punch 20. The second ironingdie 18C has a processing hole smaller than the first ironing die 18B,and the third ironing die 18D has a processing hole even smaller thanthe second ironing die 18C. This makes the peripheral wall portionflatten out in succession, and makes the thickness even thinner, andalso increases the hardness by work hardening. When the cup bodyfinishes passing through the third ironing die 18D, a prismatic batterycase 9 of desired shape is completed. In this case, DI processing isconducted on an intermediate cup body 8 of substantially ellipticalshape with a small minor-axis/major-axis ratio. In other words, it isconducted on an intermediate cup body 8 with its cross section made anelliptical shape, close to a rectangle. Consequently, it is possible toDI process the intermediate cup body 8 in a reasonable way, and tomanufacture a prismatic battery case 9 of desired shape with stability.

This prismatic battery case 9 is detached from the drawing and ironingmachine by the stripper 19. Then, its upper side portion (lug portion)is cut off because it has become a somewhat distorted shape through eachof the processings, and becomes the prismatic battery case 9 shown inFIG. 3E. There is still the jagged surface 8d left on the bottom face ofthis battery case 9, which is formed during impact molding.

In the manufacturing method of the prismatic battery case 9 of thisembodiment, a prismatic battery case 9 of desired shape is manufacturedby the first step and the second step. This first step manufactures anintermediate cup body 8 in one step of impact molding. This intermediatecup body 8 is equal to the second intermediate cup body 3 which ismanufactured by being put through the first step of deep drawing, andthe second step composed of multi-stages of redrawing in themanufacturing method of the prismatic battery case in theabove-mentioned prior application. The second step is the DI processing,which is excellent in production speed. Consequently, there areadvantages in this method over the manufacturing method of the priorapplication, in which the number of steps is reduced substantially toimprove the productivity remarkably, a prismatic battery case 9 ofdesired shape is manufactured easily, because the DI processing is doneto an intermediate cup body 8 with a cross section made substantially anelliptical shape close to a rectangle, and the dimensional precision ofthickness or the like is improved by the DI processing which molds withone stroke of action of the DI punch 20.

The prismatic battery case 9 obtained by this embodiment, shown in FIG.3E, has a uniform thickness entirely. However, the impact molding in thefirst step of this embodiment has excellent shape selectivity in whichit is possible to easily mold into a random shape by configuring the gapbetween the punch 13 and the wall of the processing hole 11 a of the die11. Accordingly, it is also possible to form the case into a shape witha cross section substantially rectangular and with the thickness at theplate portion on the shorter side 4 a made thicker than the thickness atthe plate portion on the longer side 4 b, like the prismatic batterycase 4 obtained by the manufacturing method of the prior application,shown in FIG. 5D.

It is also possible to manufacture a prismatic battery case 21 with ashape shown in the longitudinal sectional view of FIG. 3F, with ease.This prismatic battery case 21 has a thin wall portion 21 c, which has awall about ten percent thinner than the other portions, formed at thecircumference of the opening on the plate portion on the shorter side 21a, and on the plate portion on the longer side 21 b, namely, at thecircumference of the sealing portion, when it is made into a prismaticbattery. This thin wall portion 21 c can be formed by making aprescribed portion of the DI punch 13 of the drawing and ironing machinein the second step to have a shape expanding slightly to a largerdiameter.

Specific examples according to the manufacturing method of the prismaticbattery cases 9 and 21 of the embodiments described above will beexplained next. First, the measured values when the first step wasconducted will be shown. The pellet 7 is an oval shape using aluminum asits material, and the thickness is 3.6 mm, the length of the major axisis 30.9 mm, and the length of the minor axis is 9.8 mm. The intermediatecup body 8 has a thickness of 0.4 mm, the length of the major axis is31.1 mm, and the length of the minor axis is 10.0 mm. The prismaticbattery case 9 manufactured by being subjected to the second step, shownin FIG. 3E, has a thickness of 0.2 mm or less at the plate portion onthe shorter side 9 a, and at the plate portion on the longer side 9 b.The thickness of the bottom plate portion is 0.4 mm, the length of thelonger side is 29.5 mm, and the length of the shorter side is 5.3 mm.Since it went through this kind of transformation in shape, it waspossible to manufacture a prismatic battery case 9 with almost nodistorted deformation, and with high dimensional precision, smoothly.

Specific examples according to the manufacturing method thatmanufactured similar prismatic battery cases using pellets with othershapes will be explained. First, the measured values when the first stepwas conducted will be shown. The pellet is an oval shape with a crosssection of substantially rectangular shape with rounded portions at thefour corners of the rectangle, and using aluminum as its material. Thethickness of the pellet is 3.6 mm, the length of the longer side is 29.5mm, and the length of the shorter side is 5.0 mm. The intermediate cupbody has a thickness of 0.4 mm, the length of the longer side is 30.0mm, and the length of the longer side is 5.5 mm.

Then, a battery case which is similar to the prismatic battery case 9shown in FIG. 3E was manufactured by being subjected to the second step.This battery case has a thickness of 0.2 mm at the plate portion on theshorter side 9 a, and at the plate portion on the longer side 9 b. Thethickness at the bottom plate portion is 0.4 mm, the length of thelonger side is 29.5 mm, and the length of the shorter side is 5 mm.Since it went through this kind of transformation in shape, it waspossible to manufacture a prismatic battery case with almost nodistorted deformation, and with high dimensional precision, smoothly. Inparticular, it has advantages such as being able to restrain anexpansion of the bottom face in the DI process, or having small bendingand drawing rates, making it easy to conduct drawing in the DI process.This is made possible because the case is formed from a material with ashape having a cross section of substantially rectangular shape androunded portions at the four corners of the rectangle.

FIG. 4 is a longitudinal sectional view showing a prismatic lithium ionrechargeable battery that is constituted by using the prismatic batterycase 21 shown in FIG. 3F. This prismatic battery has a sealing plate 22attached to the peripheral edge inside the opening of the prismaticbattery case 21 by intermeshing. This intermeshing portion 23 betweenthe prismatic battery case 21 and the sealing plate 22 is unified bylaser beam welding, and sealed airtight and liquid tight. The sealingplate 22 is formed to be a shape with its central portion sinkinginside. It also has a through hole 24 formed thereon. A gasket 27 madeof synthetic resin coated with a sealant composed of a mixture of blownasphalt and mineral oil is mounted in unity onto this through hole 24.This synthetic resin gasket 27 is resistant to electrolyte solution andhas electricity insulating characteristics.

There is a rivet 28 made of nickel or nickel-plated steel fastened tothe gasket 27. The rivet 28 also serves as the negative electrodeterminal. This rivet 28 is inserted into the central portion of thegasket 27, and fastened by having its front-end portion caulked with awasher 29 intermeshed on its lower part, and is closely adhered to thegasket 27 in a liquid tight and airtight state. This gasket 27 of thisembodiment is formed in unity with the sealing plate 22 by injectionmolding. There is an exhaust hole 30 of substantially elliptical shapeprovided between the rivet 28 serving as the negative electrodeterminal, and the outer edges on the major axis side of the sealingplate 22. This exhaust hole 30 is obturated by an aluminum foil 31,which is attached by pressure and unified to the inner surface of thesealing plate 22 to form an explosion proof safety vent.

There is an electrode group 32 accommodated in the accommodating unitfor the power generation elements in the prismatic battery case 21. Thiselectrode group 32 is formed by having one positive electrode plate (notshown) and one negative electrode plate (not shown) wound around with aseparator 33 composed of microporous polyethylene film interposedtherebetween. This electrode group 32 is wrapped around its outermostcircumference by the separator 33, and its cross section is formed to bean elliptical shape. A positive electrode lead 34 of the electrode group32 is connected to the inner surface of the sealing plate 22 by spotwelding of laser beam, and a negative lead plate 37 of the electrodegroup 32 is connected to the washer 29 by resistance welding.

A liquid pouring hole 38 is provided in the sealing plate 22, and aprescribed amount of organic electrolyte solution is poured through thisinjection hole 38. After the pour, the liquid pouring hole 38 is coveredby attaching and fixing a cover plate 39, and then the cover plate 39and the sealing plate 22 are welded together by laser beam to completethe prismatic battery. The electrode group 32 was explained for the casein which it was wound around to make a cross section of ellipticalshape. However, this prismatic battery case 21 can be applied to a casein which a prismatic battery is constituted by accommodating anelectrode group constituted by stacking a plurality of positiveelectrode plates and a plurality of negative electrode plates withseparators interposed therebetween, as in a common prismatic cell.

This prismatic battery is constituted by using the prismatic batterycase 21 that is manufactured by the manufacturing method of theembodiments mentioned above. Since the prismatic battery case 21 ismanufactured in a small number of steps, productivity is improvedaccordingly. The prismatic battery case 21 is formed with high precisionin its dimensions such as the thickness by the DI method. Thus, when thethickness of this prismatic battery case 21 is made to be as thin aspossible, this prismatic battery will have sufficient pressure resistingstrength, while pursuing an improvement in energy density per volume.This kind of effect is achieved similarly in the case where theprismatic battery case 9 shown in FIG. 3E is used. However, in a casewhere the prismatic battery case 21 is used, the sealing plate 22 issupported by the tier portion between the thin wall portion 21 c of theprismatic battery case 21 and the other portions during the laser beamwelding of the intermeshed portion 23 between the sealing plate 22 andthe prismatic battery case 21. There is an advantage in that laser beamwelding is carried out easily without any need of means for supportingthe sealing plate 22.

A manufacturing method of a prismatic battery case according to a secondembodiment of the invention will be explained next. In thismanufacturing method, an intermediate cup body 8 similar to the ones inthe first embodiment is molded by impact molding in the first step,shown in FIG. 1. This intermediate cup body 8 is DI processed by thedrawing and ironing machine in the second step, shown in FIG. 6. In FIG.6, elements that are the same or similar to the elements in FIG. 2 willbe given the same reference symbols, and redundant explanations will beomitted. The only difference between this drawing and ironing machine,and the one shown in FIG. 2 is the constitution of the DI punch 40. Thatis, the DI punch 40 has an outer shape of prismatic plate form with across section of substantially rectangular shape corresponding to theprismatic battery case to be manufactured, as shown in a perspectiveview of FIG. 7A and FIG. 7B, that is an enlarged view of the VIIBportion of FIG. 7A. There are also processing grooves 41 of lattice formformed on a portion starting from the bottom end up to a prescribedlocation, on both of the side surfaces on the longer sides. Theselattice formed processing grooves 41 are communicatively connected toeach other through the intervention of intersections 42, where theyintersect with each other.

The DI processing in this second step is basically similar to the secondstep of the first embodiment, and only the dissimilar points will beexplained. When the intermediate cup body 8 passes through the drawingdie 18A and the first and second ironing dies 18B and 18C, and the drawnand ironed cup body 36 passes through the third ironing die 18D, theinner surface of the cup body 36 is pressed and made strong contact withthe outer surface of the DI punch 40 by the pressurizing force of thesmallest ironing processing hole of the third ironing die 18D, as shownin FIG. 8. With this pressing contact, a portion of the material on theinner surface side of this cup body 36 is pushed into the processinggrooves 41 of the DI punch 40, while being subjected to plasticdeformation. This transcribes the processing grooves 41 onto the innersurface of the cup body 36, and convex protruding portions 43 of latticeform is formed, corresponding to the processing grooves 41.

When the convex protruding portions 43 are formed on the inner surfaceof the cup body 36, the material on the inner surface side of the cupbody 36 moves in unity with the DI punch 40 with hardly any processingbeing done, because it is put in a state with resistance added betweenitself and the DI punch 40. It is also possible to make the flow of thematerial smoother, by restraining the material surplus phenomenon fromhappening between the third ironing die 18D and the DI punch 40, becausethe material on the outer surface side of the cup body 36 is ironedmainly by the third ironing die 18D. It is also possible to restrain theprocessing speed at the plate portion on the longer side, thereby makingthe processing speed constant as a whole, because the convex protrudingportions 43 are made to be formed on only the plate portion on thelonger side of the cup body 36.

As a result, it is possible to manufacture a prismatic battery casehaving a uniform thickness without any corrugation on either the innerside or the outer side of the case with this manufacturing method. Inother words, when, for instance, the cup body 36 is DI processed with aDI punch of prismatic plate form with its side face on the longer sidemade planar, the material on the inner surface side and the outersurface side of the cup body 36 are not processed by the DI punch andthe ironing die. Thus, the material surplus phenomenon will occur, andcorrugations, in which a thickness thinner than the clearance betweenthe DI punch and the die is induced at part of the places, will begenerated. When DI processing a prismatic battery case, the processingspeed at the plate portion on the longer side becomes faster than thespeed at the plate portion on the shorter side, and the plate portion onthe longer side is stretched and becomes thinner. However, this kind ofproblem is dissolved all at once, by adopting the manufacturing methodof this embodiment.

FIG. 9A is a perspective view showing the longitudinal sectional shapeof a prismatic battery case 44 obtained by the manufacturing method ofthe second embodiment, and FIG. 9B is a perspective view showing aportion of the battery case 44 in enlarged form, and FIG. 9C is anenlarged sectional view of one portion. This prismatic battery case 44has an outer shape of prism form with a bottom, and has a cross sectionof rectangular shape. There are also multitudes of convex protrudingportions 43 of lattice form, which are similar to the lattice form ofthe processing grooves 41 of the DI punch 40, formed on the innersurface of the case at the plate portion on the longer side 44 a. Theseconvex protruding portions 43 are mutually connected through theintervention of the intersections 47.

This prismatic battery case 44 is made to be as thin as possible, but ithas enough strength to effectively restrain an expanding deformationwith the lattice formed convex protruding portions 43 functioning justlike reinforcement crosspieces to provide extremely high pressureresisting strength. The outer surface and the inner surface of the caseare both made to be planes of high precision with no corrugations. Sincethis prismatic battery case 44 has convex protruding portions 43 linkedto each other by the intervention of the intersections 47, there are twodirections in which its strength is increased by the convex protrudingportions 43, and even stronger pressure resisting strength is achieved.

Specific explanations will follow, with references to FIG. 9C. It ispossible to achieve a prescribed effect of effectively restraining theexpanding deformation due to a rise in pressure within the battery,while maintaining sufficient energy density, when the convex protrudingportions 43 are formed on the inner surface of the battery case, withthe height H, width W, and interval K of the protrusion configured to bewithin the following range of values.

That is to say, it is preferable that the protruding height H of theconvex protruding portion 43 is configured to be five to fifty percentof the thickness D of the battery case (the thickness of the plateportion on the longer side 44 a in a prismatic battery). When it is lessthan five percent, there is no restraining effect on the expandingdeformation. When it is fifty percent or more, the capacity of thebattery case decreases, and this not only incurs a reduction in energydensity per volume, but also makes it difficult to manufacture the case,per se. A more preferable protruding height H is to configure it to be avalue within a range of five to twenty percent of the thickness D, andthe most preferable protruding height H is to configure it to be a valuewithin a range of five to ten percent of the thickness D. A specificnumerical value is 0.01 mm to 0.02 mm.

It is preferred that the width W of the convex protruding portion 43 beconfigured to be within a range of one to thirty times the protrudingheight H. When it is one time or less, it is not possible to form convexprotruding portions 43 that have enough protruding height H toeffectively restrain the expanding deformation. When it is thirty timesor more, the inner capacity of the battery case decreases, and incurs areduction in energy density per volume. A more preferable width W is toconfigure it to be a value within a range of five to twenty times theprotruding height H, and the most preferable width W is to configure itto be a value within a range of ten to fifteen times the protrudingheight H.

It is preferred that the interval K of the convex protruding portion 43be configured to be within a range of two to twenty times the width W.When it is two times or less, the inner capacity of the battery casedecreases, and incurs a reduction in energy density per volume. When itis twenty times or more, the restraining effect of the expandingdeformation becomes insufficient. A more preferable interval K is toconfigure it to be a value within a range of five to fifteen times thewidth W.

FIG. 10A is a view of the intermediate cup body 8 manufactured by beingsubjected to the first step of the manufacturing method of the secondembodiment, seen from the opening. FIG. 10B is a view of the prismaticbattery case 44 manufactured by being subjected to the second step, seenfrom the opening. When T1 is the thickness of the plate portion on thelonger side 8 a, T2 is the thickness of the plate portion on the shorterside 8 b, and T3 is the thickness of the corner portion 8 c, theintermediate cup body 8 of FIG. 10A has a shape with their relativerelation to be T1<T2<T3. To be specific, the thickness T1 of the plateportion on the longer side 8 a is 0.40 mm, the thickness T2 of the plateportion on the shorter side 8 b is 0.55 mm, and the thickness T3 of thecorner portion 8 c is 0.75 mm. The thickness of the bottom plate portionis 0.40 mm. The first impact molding has excellent shape selectivity inwhich it is possible to easily mold a random shape by configuring thegap between the punch 13 and the wall of the processing hole 11 a of thedie 11, as mentioned above. Consequently, it is possible to manufactureintermediate cup bodies 8 with different thicknesses T1 to T3 at therespective portions, easily and quickly in one step.

An intermediate cup body 8 with the aforementioned shape has all theportions including the plate portion on the longer side 8 a, the plateportion on the shorter side 8 b, and the corner portion 8C except thebottom plate portion ironed at the same rate when being DI processed atthe next step. Thereby, it is possible to obtain a battery case 44 witha prescribed thickness of design, while positively preventing anoccurrence of problems such as ruptures or tear-offs.

When t1 is the thickness of the plate portion on the longer side 44 a,t2 is the thickness of the plate portion on the shorter side 44 b, andt3 is the thickness of the corner portion 44c, the intermediate cup body44 of FIG. 10B maintains a shape with their relative relation to bet1<t2<t3, similar to the intermediate cup body 8. To be specific, thethickness t1 of the plate portion on the longer side 44 a is 0.20 mm,the thickness t2 of the plate portion on the shorter side 44 b is 0.30mm, and the thickness t3 of the corner portion 44 c is 0.50 mm. This isa result of having the whole of the peripheral side surfaces of theintermediate cup body 8 molded to be almost uniformly thinner by the DIprocessing. The thickness of the bottom plate portion remains to be 0.40mm, which is the same as the intermediate cup body 8.

Consequently, when manufacturing beforehand an intermediate cup body 8having thicknesses T1 to T3 by impact molding, the controlling to obtaina prismatic battery case 44 with prescribed thicknesses t1 to t3 becomeseasy, and the burden of processing also becomes less in the DIprocessing. In this case, the thicknesses T1 to T3 have ratioscorresponding to the ratio of the thicknesses t1 to t3 at each of theportions of the prismatic battery case 44 of desired shape. The bottomplate portion is molded to a prescribed thickness when the case isimpact molded in the first step. Since, this thickness does not changethrough the DI processing, the bottom plate portion is formed to be aprescribed thickness in as few numbers of steps as possible.

In a battery case 44 having the shape mentioned above, when the pressureinside the battery rises, the force that tries to expand and deform theplate portion on the longer side 44 a in the outward direction, and theforce that tries to sink the plate portion on the shorter side 44 b inthe inward direction are both acted upon the case simultaneously.Consequently, the plate portion on the shorter side 44 b, which has thethickness t2 thicker than the plate portion on the longer side 44 a,effectively prevents the plate portion on the longer side 44 a fromexpanding and deforming in the outward direction. The plate portion onthe longer side 44 a tries to expand and deform in the outward directionusing the corner portion 44 c just like a fulcrum. Hence, the cornerportion 44 c, which has the thickness t3 made even thicker than theplate portion on the shorter side 44 b, effectively prevents the plateportion on the longer side 44 a from expanding and deforming in theoutward direction. forming the convex protruding portions 43 of latticeform.

INDUSTRIAL APPLICABILITY

With the prismatic battery case and its manufacturing method of thepresent invention, it is possible to manufacture a prismatic batterycase that has sufficient strength to effectively restrain expansion anddeformation due to a rise in pressure within the battery, even whenmaking the thickness of the plate portion on the longer side as thin aspossible. It is also possible to manufacture, with good productivity, aprismatic battery case that improves both the energy density per volumeand the energy density per weight, in a prismatic battery case with highspace utilizing efficiency. Consequently, it is suited for using in aprismatic battery that requires a reduction in size and thickness of thebattery case.

1. A method for manufacturing a prismatic battery case, comprising: afirst step for molding an intermediate cup body by impact molding apellet, wherein said pellet has a shape that, in a plan view,corresponds to an outline of a cross sectional shape of the prismaticbattery case; and a second step for molding a prismatic battery casewith a cross section of a substantially rectangular shape by DIprocessing said intermediate cup body, the DI processing conductingdrawing and ironing continuously in one action said intermediate cupbody having a bottom face, said intermediate cup being molded by impactmolding said pellet using a punch having a jagged surface along afront-end surface, said jagged surface being perpendicular to an axisline of the punch, the punch being adapted for forming a jagged surfacethroughout said bottom face of said intermediate cup so that the punchand pellet maintain a predetermined positional relationship during saidfirst step.
 2. The method for manufacturing a prismatic battery caseaccording to claim 1, wherein: the intermediate cup body is molded byimpact molding the pellet to have a prismatic shape with a bottom, and across section of rectangular shape that satisfies a relation ofT1<T2<T3, where thicknesses of a plate portion on a longer side, a plateportion on a shorter side, and a corner portion of the cross section aredefined as T1, T2, and T3, respectively; and the prismatic battery easeis molded by molding said intermediate cup body with the DI processingto have a shape with a cross section of rectangular shape that satisfiesa relation of T1<T2<T3, where thicknesses of a plate portion on a longerside, a plate portion on a shorter side, and a corner portion of thecross section are defined as T1, T2, and T3, respectively.
 3. The methodfor manufacturing a prismatic battery case according to claim 2,wherein: the intermediate cup body is molded by impact molding thepellet to have a shape with a ratio of the thickness of a bottom plateportion to the thickness (T1) of the plate portion on the longer sidebeing 0.6 to 1.3, and a ratio of the thickness of the bottom plateportion to the thickness (T2) of the plate portion on the shorter sidebeing 1.0 to 1.8.
 4. A prismatic battery, comprising: a power generationelement including an electrode group and electrolyte solution isaccommodated inside a prismatic battery case manufactured by themanufacturing method according to claim 1, and an opening of the case isliquid sealed with a sealing plate; said prismatic case having a bottomsurface, wherein said bottom surface has a jagged contour throughout. 5.A prismatic battery case, comprising: an intermediate cup body, saidintermediate cup body having a bottom face, said intermediate cup beingmolded by impact molding a pellet using a punch having a jagged surfacealong a front-end surface thereof, said jagged surface beingperpendicular to an axis line of the punch, the punch forming a jaggedsurface throughout said bottom face of said intermediate cup so chat thepunch and pellet maintain a predetermined positional relationship; saidpellets being molded to have a prismatic shape with a bottom and, across section of rectangular shape chat satisfies a relation ofT1<T2<T3, where thicknesses of a plate portion on a longer side, a plateportion on a shorter side, and a corner portion of the cross section aredefined as T1, T2, and T3, respectively; and the prismatic battery caseis molded by molding die intermediate cup body with DI processing tohave a shape with a cross section of rectangular shape that satisfies arelation of T1<T2<T3, where thicknesses of a plate portion on a longerside, a plate portion on a shorter side, and a corner portion of thecross section are defined as T1, T2, and T3, respectively, the DIprocessing conducting drawing and ironing continuously in one action. 6.A method for manufacturing a prismatic battery case, comprising: a firststep for molding an intermediate cup body by impact molding a pellet ofa prescribed shape; and a second step for molding a prismatic batterycase with a plurality of convex protruding portions formed in latticeform arrangement extending in linear form on the inner surface of thecase, and formed on at least the place portions on the longer sides, soas to be thick in a thickness direction of the case by DI processingsaid intermediate cup body using a DI punch, the DI processingconducting drawing and ironing continuously, in one action, the DI punchhaving processing grooves of lattice form formed on at least surfaces onlonger sides of a prismatic plate with a cross section of rectangularshape.
 7. A prismatic battery, comprising: a power generation elementincluding an electrode group and electrolyte solution is accommodatedinside a prismatic battery case manufactured by the manufacturing methodaccording to claim 6, and an opening of the case is liquid sealed with asealing plate.